It is known that when such machines are of the type having inductive plasma exciters they enable much higher ion densities to be obtained, of the order of 100 times higher than machines of the capacitive excitation type.
For example, Document WO 86/06923 discloses a machine of the inductive exciter type in which the exciter antenna couples radio-frequency energy to a cylindrical volume, and over a length greater than 200 mm.
For the excitation source, that layout provides high ion density at low pressures, while the radial profile of the ion density in the source or close to the plasma-forming zone is not uniform.
In order to produce a uniform ion density profile over the substrate to be treated, the substrate is placed remote from the source in a diffusion plasma region. The plasma has an ion density that is low, or extremely low for electronegative gases such as SF.sub.6, C.sub.2 F.sub.6 or Cl.sub.2 for which density drops rapidly as a function of pressure (below about 10.sup.-3 mbars), and of distance between the source and the substrate. Such gases are typically used at pressures greater than 10.sup.-2 mbars for deep fast etching or for PECVD (Plasma Enhanced Chemical Vapor Deposition).
Document EP 0 431 282 also discloses a machine of the inductive exciter type in which the exciter antenna is plane and has the shape of a spiral coil.
That layout provides good ion uniformity for the excitation source. Compared with other known inductive sources, it provides better uniformity at high pressures (greater than 10.sup.-2 mbars). Compared with other known inductive sources, it also provides better ion densities when electronegative gases are used such as SF.sub.6, C.sub.2 F.sub.6, and Cl.sub.2, as mentioned above.
However, with such an antenna as the plasma excitation source, the electromagnetic field is relatively strong at a distance remote from the antenna, thereby imparting high energy to the ionized particles, and, in the case of etching, this might damage the substrate to be etched.
Document EP 0 403 418 describes high density plasma etching and deposition apparatus in which the exciter antenna is constituted by two circular loops situated one above the other and parallel to the substrate to be treated.
The two loops are interconnected by conductors and the resulting assembly is fed by a radio-frequency power generator. The antenna is disposed around a dielectric tube made of quartz.
However, with such a layout, high levels of erosion have been observed on the tube where the conductors interconnect the two loops.